1. Field of the Invention
The present invention relates to a sheet feeding apparatus and an image forming apparatus having the same.
2. Description of the Related Art
An image forming apparatus for forming an image on a sheet such as a printer, a copying machine, or a fax machine includes a sheet feeding apparatus for feeding a sheet to an image forming unit. Such a sheet feeding apparatus includes an apparatus having a unit with a function of sensing a stacking amount of sheets stacked in the device.
In Japanese Patent Application Laid-Open No. H08-259039, an image forming apparatus is disclosed which includes a cassette for stacking sheets, a sheet feed roller for feeding the sheets stacked in the cassette, driving means for driving the sheet feed roller, and sensing means for sensing that a sheet fed by the sheet feed roller reaches a predetermined point on a conveyance path. In a case where sheet feeding is not performed, a distance between the sheet feed roller and the uppermost sheet in the cassette is changed depending on the stacking amount of sheets in the cassette. Therefore, a time taken to sense the front end of the sheet by the sensing means after a sheet feed signal is output is changed depending on the stacking amount of the sheets. In addition, the image forming apparatus detects the stacking amount (remaining amount) of the sheets by measuring the time taken to sense the sheets by the sensing means after the driving means drives the sheet feed roller.
In Japanese Patent Application Laid-Open No. 2006-137564, a sheet feeding apparatus is disclosed which elevates a pressure plate to lower the position of an uppermost surface of sheets from feeding means by a constant amount at substantially the same timing regardless of a stacking height of the sheets stacked on the pressure plate. The sheet feeding apparatus is able to maintain a constant distance between the uppermost surface of the sheets and the feeding means regardless of a stacking amount of the sheets when the feeding means is not operated. In addition, when a sheet feeding operation is started, the pressure plate is operated so as to allow the uppermost surface of the sheets stacked in the pressure plate to come in pressure contact with the feeding means. Since an operation distance of the pressure plate is constant during the sheet feeding operation, a sheet conveyance timing can become uniform regardless of the stacking amount of the sheets. Accordingly, stability of sheet feeding is enhanced.
However, the mechanism for detecting the remaining amount of sheets disclosed in Japanese Patent Application Laid-Open No. H08-259039 may not be applied to the sheet feeding apparatus disclosed in Japanese Patent Application Laid-Open No. 2006-137564. This is because although the stacking amount of the sheets is detected on the basis of the time taken to sense the sheets by the sensing means after the sheet feed roller is driven in Japanese Patent Application Laid-Open No. H08-259039, the sheet feeding apparatus disclosed in Japanese Patent Application Laid-Open No. 2006-137564 is configured to uniformize the time.
In addition, if hardware such as a dedicated switch or sensor is additionally provided to detect the stacking amount of the sheets, there is a problem in that mechanical or electrical costs are increased.